Blackleg

Blackleg affects cattle worldwide and is caused by Clostridium chauvoei. This infection begins when the susceptible animal ingests the endospores. The endospores then cross over the gastrointestinal tract and enter the bloodstream. Endospores are deposited in tissue throughout the animal’s body. They lie dormant in the tissue until they become activated and trigger the disease. This bacterium is activated in an anaerobic (oxygen deficient) environment.

This means it requires a site where there is low oxygen content, such as damaged, devitalized or bruised tissue. Muscle tissue that has been damaged will have a compromised blood flow; as a result, oxygen will not be as readily delivered to the affected area. Therefore, any activity that causes bruising can promote the disease. Once this stimulating event occurs (transporting, handling, injection sites, rough/rigorous pasture activity), the spores germinate and multiply into the disease-causing bacteria. Disease can develop without any history of wounds to the animal.

Although blackleg has occurred in calves as young as two months of age, the disease generally affects animals between six months and two years of age. Occasionally, losses may be seen in adult cattle. Blackleg infections typically occur during the late summer and early fall, and blackleg usually affects rapidly growing calves.

It may be more prevalent on farms that have recently dug a waterline through a pasture or done some type of excavation. It may also be noted in flooded areas (this allows the spores to rise to the surface of the ground). It may also occur during times of drought when the grass is very low, and cattle will ingest spores from the soil during grazing.

Typically, animals infected with blackleg die rapidly without any signs of illness. However, clinical signs that may be noted very early in the disease include lameness, loss of appetite, fever and depression.
Animals quickly die within 12 to 48 hours after contracting the disease. Although treatment usually fails, appropriate doses of penicillin may prove helpful. If an animal does survive, it will likely suffer from a permanent deformity.

Lesions in a dead animal associated with blackleg include swelling of the affected muscle tissue (legs, neck, hip, chest, shoulder, back or elsewhere). The swelling is due to fluid accumulation as well as gas buildup, which are produced by the infectious bacteria. When pressure is applied to the affected areas, gas can often be felt moving while producing a crackling sound under the skin. As seen in the picture below, affected muscle tissue will contain dark areas of necrotic tissue, hence the name blackleg. This affected tissue may also have a foul odor (usually described as rancid butter).

Vaccination is the only way to effectively control this disease. It is generally recommended to vaccinate calves between two and three months of age. Before this period, calves should be protected through passive transfer of antibodies from their dam’s colostrum. A regular vaccination protocol should be followed at weaning. Calves should receive vaccine dosages according to the manufacturer’s label. Some vaccines require one injection followed by a booster in 2 to 6 weeks. Other vaccines are now approved for a single dose injection.
Always be sure to read and follow the instructions on the label when using any vaccine. Blackleg vaccines should be administered subcutaneously (under the skin) in the neck area. The common blackleg vaccines are referred to as “7-way” because they protect against other clostridial diseases such as malignant edema, black disease, enterotoxemia, etc.